The invention relates to a method of regulating and stabilizing an AlF.sub.3 content, which is at least about 10% by weight, in the bath of an electrolysis cell for the production of aluminum from alumina dissolved in a cryolite melt.
In an electrolysis cell for the production of aluminum, a bath or an electrolyte is used which consists essentially of cryolite, a sodium aluminium fluorine compound (3NaF.AlF.sub.3). In addition to the alumina to be dissolved, especially substances which lower the melting point are also added to this cryolite, for example aluminum trifluoride AlF.sub.3, lithium fluoride LiF, calcium difluoride CaF.sub.2 and/or magnesium difluoride MgF.sub.2. Thus, a bath in an electrolysis cell for the production of aluminum contains, for example, 6 to 8% by weight of AlF.sub.3, 4 to 6% by weight of CaF.sub.2 and 1 to 2% by weight of LiF, the remainder being cryolite. Depending on the content of the additives, the melting point of the bath is lowered in this way to the range from 940.degree. to 970.degree. C., which is the industrially used temperature range.
However, bath additions have not only positive effects such as, for example, a lowering of the melting point, but frequently also have negative effects. For example, the addition of lithium fluoride does not allow foil qualities for capacitors to be obtained without special treatment of the metal.
Within the scope of the present invention, the only baths of interest are baths with additions of AlF.sub.3, which is a Lewis acid, leading to an excess of at least 10% by weight. This excess is expressed as the NaF/AlF.sub.3 molar ratio or weight ratio including the cryolite, or as the percentage content of the excess of free AlF.sub.3. The second variant is selected for the text which follows, as already indicated by the above numerical examples.
By means of the addition of AlF.sub.3 the liquidus line of the ternary cryolite/alumina/aluminum trifluoride system can be lowered according to a square law. An addition of 10% by weight of AlF.sub.3 effects a lowering of the temperature by about 25.degree. C. Because of the known square dependence on the concentration, it is an obvious aim to operate with higher concentrations of aluminum fluoride, in particular since further advantages have also been recognized:
Because of the lower temperature, the bath components are less aggressive, thereby the service life of the electrolysis cell can be extended. Moreover, the anode consumption can be kept lower, which has an additional effect on the economics.
Less aluminum dissolves in the electrolyte, which means a higher current yield.
The molten metal contains less sodium, which reduces the service life of the cathode.
It has also been shown, however, that the lowering of the bath temperature by a high AlF.sub.3 content has not only advantages, but that resulting disadvantages also have to be accepted:
The solubility of alumina in the electrolyte is reduced.
The electrical conductivity of the bath decreases with increasing AlF.sub.3 content and decreasing temperature. The stability of the solidified side bank decreases.
The solubility of aluminum carbide increases steeply with increasing AlF.sub.3 content. As a result, above all the three-phase zone (carbon lining, electrolyte, molten metal) is impaired, especially if there is no protection by solidified electrolyte material. Moreover, dissolved aluminum carbide migrates to the anode and lowers the current yield by reaction.
Sodium ions are charge carriers of the electrolysis current, whereas the aluminum ions are reduced at the cathode. Therefore, a high NaF/AlF.sub.3 ratio arises in this region, which can lead to the solidification of electrolyte material.
Furthermore, in addition to these known disadvantages, it has been found that, at an AlF.sub.3 content at or above 10% by weight, fluctuations of a wavelength of several days, for example 10 to 30 days, can arise in the bath. During this period, the AlF.sub.3 content fluctuates slowly within wide limits, for example in the range from 6 to 20% by weight.
In accordance with the abovementioned square law, these fluctuations of the AlF.sub.3 content also involve temperature fluctuations, for example in the range from 930.degree. to 990.degree. C. Moreover, an aluminum fluoride content at or above 10% by weight entails fluctuations in the liquid level in the range of 10-30 cm. At lower AlF.sub.3 contents below 10% by weight, no such pronounced fluctuations have been found.